Abstract The proposed studies will elucidate cellular and molecular mechanisms that organize the activities, fate, and tissue localizations of CD4+ T cells in the lungs. CD4+ T cells are instructed by antigen presentation from MHC-II and costimulatory or checkpoint molecules. Our preliminary data reveal that MHC-II is widely expressed by lung epithelial cells, and dynamically regulated during and after resolution of pneumococcal pneumonia. Furthermore, we observe that distinct sub-types of lung epithelial cells express different patterns of costimulatory and checkpoint molecules. We propose that epithelial cell MHC-II pairs with cell-type-specific expression of costimulatory and checkpoint molecules to appropriately instruct CD4+ T cells, localizing CD4+ T cell activities and all aspects of adaptive immunity that depend on lung CD4+ T cells near more forgiving regions (conducting airways) and away from more fragile and demanding elements (alveoli), to increase immune resistance while diminishing lung injury. Our central hypothesis is that MHC-II on the lung epithelium drives adaptive immunity toward the conducting airways and away from the alveoli, to be tested by pursuing the following specific aims: (1) Test the hypothesis that pulmonary epithelial cells direct the anatomical segregation of CD4+ T cells in the lung. This will be accomplished using multiple microscopy approaches to document the regional kinetics and epithelial interactions of CD4+ cell expansion and contraction during and after pneumonia, RNAseq of epithelial cells that are bright vs. dim for MHC-II, and conditional mutation of MHC-II throughout all the lung epithelium. (2) Test the hypothesis that alveolar epithelial type II cells use MHC- II plus PD-L1 to limit inflammatory lung injury. This will be accomplished using PD-L1 blockade, conditional mutation of MHC-II in alveolar epithelial type II cells, and RNAseq of disparate classes of type II cells with distinct patterns of MHC-II and checkpoint molecules. (3) Test the hypothesis that airway club cells and multiciliated cells use MHC-II plus CD40 to bolster antimicrobial defense. This will be accomplished using conditional mutation of MHC-II or of CD40 in both club cells and multiciliated cells. Altogether, the studies will reveal whether and how lung epithelial cells orchestrate CD4+ T cell activities during pneumonia and determine the TRM cell niche after resolution of infection. Results will identify naturally acquired immune mechanisms that prevent pneumonia in older children and young healthy adults, provide insight into antigen presentation pathways in the lung, and elucidate roles of checkpoint and costimulatory molecules in directing lung CD4+ T cell biology.